All these elections are anticipated prior to trading straddles. They are part of the rules for setting capital gains and losses under the mark-to market rules. If none of these elections applies to your trading, you can continue without making an election.
Mixed straddle is any straddle that has at least one but not all of the straddles is a 1256 contract. On the day of the first contract, it must be identified as a mixed straddle. Once election is made it cannot be revoked without IRS consent.
Straddle by straddle identification election is when positions that are held the day before this election are sold for fair market value on the last day business day before the election. This is reporting gain or loss as if mixed straddle had not been established.
Mixed straddle account election is to establish one or more mixed straddle accounts for 2018 by the due date of your 2017 tax return.
I've attached the instructions under the form to give you more detail on these elections and others:
https://www.irs.gov/pub/irs-pdf/f6781.pdf
[Edited: 03.04.2018 6:19 PM EST]
